Scalable asymmetric synthesis of a key fragment of
Bcl-2/Bcl-xL inhibitors †
Sylvain Laclef, Catherine Taillier, Christine Penloup, Aurélie Viger,
Jean-François Brı̀ère, Christophe Hardouin, Vincent Levacher
To cite this version:
Sylvain Laclef, Catherine Taillier, Christine Penloup, Aurélie Viger, Jean-François Brı̀ère, et al..
Scalable asymmetric synthesis of a key fragment of Bcl-2/Bcl-xL inhibitors †. RSC Advances,
Royal Society of Chemistry, 2014, pp.39817-39821. .
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Cite this: RSC Adv., 2014, 4, 39817
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Scalable asymmetric synthesis of a key fragment of
Bcl-2/Bcl-xL inhibitors†
Sylvain Laclef,‡a Catherine Taillier,‡a Christine Penloup,b Aurélie Viger,b
Jean-François Brière,a Christophe Hardouin*b and Vincent Levacher*a
DOI: 10.1039/c4ra07821g
www.rsc.org/advances
The asymmetric synthesis of a 1,3-diamine building block for the
elaboration of Bcl-2 and Bcl-xL protein inhibitors is described through
two key steps: (1) a highly diastereoselective aza-Reformatsky reaction, and (2) a chemoselective amination under Mitsunobu conditions.
This synthetic sequence was also demonstrated to be successfully
amenable to a large-scale synthesis.
Defects in the apoptotic processes1 play an important role in
tumour initiation, progression and chemoresistance.2 Among
the apoptosis regulator Bcl-2 family (B-cell lymphoma 2), the
anti-apoptotic Bcl-2 and Bcl-xL proteins were found to be overexpressed in many cancers.3–5 As part of a complex orchestration
to regulate cell fate, the anti-apoptotic Bcl-2 and Bcl-xL proteins
and others inhibit pro-apoptotic proteins such as BAK and BAX.
Importantly, these interactions can be antagonised by BH3-only
proteins (BAD, BIM and NOXA) possessing a single BH domain
and displaying a large hydrophobic groove with the same fold.
Consequently, the development of small molecule BH3
mimetics as inhibitors of anti-apoptotic Bcl-2 and Bcl-xL
proteins is attractive for novel anticancer therapy.6–9
A fragment-based drug design10 approach has led to the
discovery of several11 potent Bcl-2 and Bcl-xL inhibitors such as 1
in Abbott Laboratories (ABT-737, Fig. 1).12 Analogues based on
similar scaffolds were recently developed.13 In that eld of
research (Fig. 1), Servier Laboratories developed conformationally restricted isosters 2, which displayed submicromolar
activity. The tricyclic architecture was aimed at addressing both
the solubility issues and at modulating the interactions with the
hydrophobic groove of the proteins. Extensive structure–activity
relationship studies revealed the essential importance of
common diamine fragments such as 3,14 containing a 1,3diamine moiety anked by a phenylthioethyl arm, for securing
both bioavailability and the potent inhibition of Bcl antiapoptotic proteins. These outcomes highlighted that the R
isomer displayed better bioactivity than the opposite
enantiomer.
As far as the construction of diamine fragment 3 was concerned, only one chiral pool based synthesis was reported using
15
L-aspartic acid precursor. This method allowed the synthesis
of compound 3 in eight steps and with a 30% overall yield.
In this context, we endeavoured to develop a reliable access
toward diamine 3 through an alternative asymmetric synthesis
approach. The aim is eventually to achieve a exible larger-scale
synthetic sequence, en route to providing signicant amounts
of Bcl-2 protein inhibitor from the key building block 3. The
retrosynthetic approach is based on both diastereoselective
a
Normandie UNIV, COBRA, UMR 6014 et FR3038; Univ Rouen; INSA Rouen; CNRS,
IRCOF, 1 rue Tesnière, 76821 Mont Saint Aignan Cedex, France. E-mail: vincent.
[email protected]
b
Oril Industrie, 13 rue Auguste Desgenétais, 76210 Bolbec, France. E-mail: christophe.
[email protected]
† Electronic supplementary information (ESI) available: For procedures and
compound characterisation. See DOI: 10.1039/c4ra07821g
‡ These two researchers equally contributed to this project.
This journal is © The Royal Society of Chemistry 2014
Fig. 1
Structures of Bcl-2/Bcl-xL inhibitors.
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aza-Reformatsky (6 to 5) and chemoselective amination key
reactions (4 to 3, Scheme 1). First, chiral Ellman's N-tert-butanesulnamide, readily available on a large scale as both enantiomers, was selected as a versatile chiral auxiliary for the
asymmetric synthesis of amine 5.16 However, despite previous
examples reporting the use of chiral Ellman's sulnimines in
Reformatsky reactions,17 the inuence of the thioether functionality of 6 on both reactivity and diastereoselectivity remains
an open issue. Then, we sought to capitalize on the N-sulnyl
protecting group of 5, in order to perform further functional
group manipulation, like the key chemoselective amination
step on 4. We are pleased to report herein our efforts towards
the development of a scalable diastereoselective synthesis of
chiral scaffold 3, a potentially versatile and useful building
block in medicinal chemistry.
According to literature procedures, bromoacetaldehyde
acetal 7 was converted to the aldehyde precursor 8 in two steps
(Scheme 2).18 The transformation of the rather unstable aldehyde 8 into the corresponding enantiomerically pure N-(tertbutylsulnyl)imine 6 was successfully carried out with copper(II)
sulfate as the dehydrating agent in a 70% yield.16 These conditions were superior to the standard use of Ti(OEt)4, which gave 6
in only a 52% yield. It should be noted that other chiral
auxiliaries, such as (R)-1-phenylethylamine or (R)-2-methoxy-1phenylethylamine, failed to give the corresponding imines,
highlighting the robustness of the Ellman's sulnamides
approach.
Then, the sulnimine 6 was treated with an excess of the
Reformatsky reagent derived from the corresponding bromo
acetate 10 (2.2 equiv.) under Barbier's conditions (Scheme 3).19
Pleasingly, the desired product 5 was obtained in a 85% yield,
with a high diastereoisomeric ratio (d.r. > 95 : 5). Actually,
changing the reaction temperature from 50 C to 0 C had
negligible impact on d.r., although it led to lower yields in some
cases. By means of Red-Al, the methyl ester 5 was easily reduced,
to furnish the desired (R)-alcohol 4 in a 70% yield. Based on
Ellman's model, already applied to the Reformatsky reagent
originating from 10, we propose the following explanation to
account for the diastereoinduction outcome.16,17c Considering
that the Reformatsky reagent derived from methyl bromoacetate 10 exists as a monomeric C-metallated species in polar
solvents,20 a regular Zimmerman–Traxler transition state
involving a six-membered intermediate with zinc metals coordinated to the sulnyl oxygen is proposed. Then, the nucleophilic attack of the Reformatsky reagent to the Re face of imine
takes place (Scheme 3). The high diastereoselectivity obtained
demonstrates that the putative coordination between sulnimine and zinc is not disturbed by other complexing functions
such as the thioether moiety.
Two different pathways were next considered to transform
the primary alcohol 4 into the tertiary amine 11 (Scheme 4).
First, following a two-step sequence, the alcohol 4 was converted quantitatively to aldehyde 12 using the mild 2,2,6,6-tetramethyl-1-piperidinyloxyl and [bis(acetoxy)-iodo]benzene
(TEMPO-BAIB) oxidative system (Route A, Scheme 4).21 Then,
the crude aldehyde 12 underwent a reductive amination
sequence in the presence of NaBH(OAc)3 to give amine 11 in a
39818 | RSC Adv., 2014, 4, 39817–39821
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Scheme 1
Asymmetric synthesis approach of diamine fragment.
Scheme 2
Synthesis of sulfinimine intermediate 6.
Scheme 3
The key Reformatsky reaction.
Scheme 4
Completion of the synthesis of diamine fragment 3.
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Scheme 5
Inter- versus intra-molecular amination under Mitsunobu
conditions.
72% overall yield. Unfortunately, we encountered signicant
reproducibility issues due to the instability of aldehyde 12 when
attempting to scale up the reaction. To overcome these difficulties, an alternative approach (Route B) based on a one-step
Mitsunobu reaction with dimethylamine was studied.22 This
strategy led to the formation of product 11, with a respectable
yield of 43% and, more importantly, with a robust scalable
protocol (vide infra). Finally, the deprotection of the N-sulnyl
functional group of 11 was achieved under regular acidic
conditions, affording the target diamine molecule 3 in a 87%
yield. The R-absolute conguration was assigned at that stage by
comparison with the optical rotation previously reported.10
Scheme 6
RSC Advances
It is worth pointing out that the outcome of the Mitsunobu
reaction (Route B, Scheme 4) is surprising considering the low
acidity of both the primary alcohol 4 and dimethylamine
starting materials, especially in the presence of the NHSOt-Bu
moiety. Indeed, it was reported in the literature that tert-butylsulnamines 13 can react intramolecularly with a proximate
alcohol to form a ve-membered pyrrolidine ring 14 (Scheme 5a).23
In our case, the formation of a four-membered azetidine ring
should be more energetically demanding.24 Moreover, we could
demonstrate (see supporting information†) that the N-methylated precursor 15 did not react under the Mitsunobu conditions
with dimethylamine (Scheme 5b). Therefore, we assume that
the NH bond favours the formation of the phosphonium
intermediate, thereby allowing the subsequent nucleophilic
substitution to take place, even with dimethylamine having a
high pKa value. On the other hand, the formation of a phosphorane intermediate could not be ruled out.25
Next, we embarked on a larger scale synthesis of diamine
target 3 by optimizing our validated sequence, with special
attention paid to minimize the number of purication procedures initially required at each reaction step (Scheme 6).
Subsequent to the easy formation of sulphide 16 on a 2 kilogram scale, the acetal deprotection into aldehyde 8 was performed with H2SO4, in order to prevent the use of corrosive HCl.
Keeping the green solvent MeTHF as the reaction media, the
formation of imine 6 was conveniently carried out by means of a
Dean–Stark distillation in the presence of the so PPTS acid.
This allowed the formation of sulnimide 6 in a 68% crude yield
(see supporting information†) on a kilogram scale through
three telescoped steps (7 to 6). Though the purity of the product
Scale-up synthesis of diamine building block 3 fumarate salt.
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was estimated to be only 69% by means of HPLC analysis, this
quality turned out to be sufficient for the subsequent steps.
Disappointingly, a solvent screening revealed that the next
Reformatsky reaction led to partial conversions in MeTHF
solvent. Further optimisation and switching to THF demonstrated that imine 6 was completely transformed into amine 5
with a high diastereoisomeric ratio of 94 : 6, aer a so citric
acid work-up to preserve the chiral auxiliary. It is worth noting
that, according to a literature procedure,17d the activation of zinc
metal by DIBAL-H is preferred to avoid the initially uncontrollable exothermicity during the Reformatsky's reagent formation. A column chromatography on silica gel improved the
purity of product 5 from 68% to 82%, as estimated by HPLC
analysis, which was found to be sufficient for the next step. The
reduction of the ester group by Red-Al (5 to 4) and the subsequent Mitsunobu reaction were next successfully telescoped in
the same solvent to furnish the crude amine 11. A silica gel
column chromatography was required to remove the large
amount of triphenylphosphine side product, and to allow the
isolation of amine 11 in a good 64% yield and more than 99%
purity, as measured by HPLC. Moreover, the isolation of the
pure major diastereoisomer of 11 was secured at this stage. The
chiral auxiliary was removed by HCl in methanol, and the corresponding diamine 3 was obtained in toluene solution aer
neutralization. Then, the nal product 17 was conveniently
isolated as a solid fumarate salt which furnished a pure material
in 99.5% ee. It is worth noting that all attempts to perform the
one-step deprotection of sulnamine 11 by fumaric acid were
unsuccessful.
Conclusions
A novel asymmetric synthesis of an enantiopure 1,3-diamine 3,
a key fragment of potent Bcl-2/Bcl-xL protein inhibitor, was
accomplished in seven linear steps. The two key steps involve
both a highly diastereoselective aza-Reformatsky reaction on a
chiral sulnimine and a chemoselective Mitsunobu reaction
allowing the introduction of the dimethylamine moiety. This
laboratory synthesis of diamine 3 was demonstrated to be
amenable to a larger-scale process up to the kilogram scale for
some steps and required only two purications by column
chromatography. Both enantiomers of diamine 3 were shown to
be available as useful building blocks of bioactive material.
Acknowledgements
This work has been partially supported by INSA Rouen, Rouen
University, CNRS, EFRD, Labex SynOrg (ANR-11-LABX-0029)
and Région Haute-Normandie (CRUNCH network). We warmly
thank J.-P. Lecouvé, the pilot plant team and the analytical team
for their valuable support.
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